Any one out there know how I work this math question out?

[thames]

This is a pure maths question:

I want to find out the gear reduction value for an electric motor motor which runs at 3500 r.p.m.( motor shaft diameter is .75mm)
I want to to power a vehicle that has 200mm wheels diameter and I want the vehicle to travel at no more than 6km per hour .

Please what is the math to work this one out.

Thats a motor speed of 3500rpm (.75mm motorshaft) gearbox reduction should be ??? This is the question what reduction should it be???to drive a 200mm diameter wheel on a vehicle so the vehicle travels at a maximum of 4km per hour

Any smart ones out there please can give me the answer or how do I work it out I was hopeless at maths at school

Thank you.

[HWKE]

3500 rpm = 210000 revs per hour. If the wheels were turned at this speed, the distance covered in one hour would be 3500*60*0.2*pi metres = 131947 mteres. since you want to limit the speed to 4000 metres per hour, the gear reduction would be 4000/131947 =1/33 approximately.
For a reduction to 6000metres/hour the ratio would be 1/22.